Structures submerged in marine ecosystems may be subjected to “biofouling”, the undesired accumulation of microorganisms, algae and/or marine animals on the surface of the structure. The process of surface biofouling may be initiated by the adhesion of a film containing organic matter and dissolved macromolecules, such as polysaccharides, proteins and protein fragments, which is then followed by the adhesion of bacteria, protozoa, microphytes, algae and invertebrates.
The accumulation of biological material can obviously become highly detrimental: a layer of marine organisms on a ship's hull can, for instance, decrease the speed and maneuverability of the ship and can also raise propulsive fuel consumption by as much as 30 percent. To mitigate these effects and to prevent the formation of such a fouling layer, marine structures have generally been coated with anti-fouling paints incorporating compounds, such as organo-tin compounds and cuprous oxide, which are toxic at low concentrations to most fouling organisms. Unfortunately, these biocidal compounds persist in water and have been linked to deformity in non-fouling marine organisms. As such, a number of States have legislated against the use of organo-tin compounds in anti-fouling paints and supranational organizations such as the Marine Environment Protection Committee (MEPC) of the International Maritime Organization have also passed resolutions of equivalent effect.
The alternatives to tin-based anti-fouling paints which were consequently developed may broadly be categorized into low-leach-rate paints, optionally having a toxic compound or moiety therein, and non-toxic fouling release paints.
The first category of paints comprises an ablative resin which slowly but constantly dissolves, taking with it the early stages of fouling. To supplement this effect, copper-based or alternative biocidal compounds may be held within the resin and released over time. Alternatively, labile functional groups within the ablative resin may hydrolyze to release a toxic compound. Problematically, these ablative coatings require a constant water flow and are most effective at consistently high water velocities. Moreover, the lifetime of these coatings is highly variant, depending on the initial thickness of the coating and the temperature of the seawater.
The non-toxic fouling release category of paints—with which the present application is concerned—are intended to prevent the attachment of fouling. Where they do foul, the strength of the attachment between the matter and the marine structure should be so weak that it can be broken either by the weight of the fouling or by the motion of the ship through the water.
The performance of the fouling release coatings is mainly determined by three properties: surface energy, on the basis that a low surface energy will discourage the initial attachment of fouling; the modulus of the coating as this influences the fracture mechanism by which the interface between a coating and a foulant will fail; and, the thickness of the coating as it can be determinative of the balance between shear and peel fracture of the applied paint or coating. Whilst micro-scale and nano-scale structuring of the coating can influence these properties, the chemistry of the coating is a critical determinant of its surface energy and modulus.
Fouling release paints based on poly(ethylene glycol), polyvinyl acetate (PVA), polyacrylates, oligosaccharides, fluorinated polymers and silicone polymers are well-known. Recently however attention has been directed to coatings which contain zwitterionic moieties. Such species can be effective at preventing non-specific protein adhesion and whole organism fouling but, as a flip-side to these effects, zwitterionic molecules are highly hydrophilic and thus have a tendency to dissolve too rapidly in water. Consequently, the prior art has been concerned with immobilizing the zwitterionic moieties within anti-fouling and fouling resistant coatings and, in particular, in incorporating said moieties into copolymers.
US Patent Application Publication No. 2008/181861 (Jiang et al.) describes a substrate, such as a ship's hull, having a low fouling surface, said surface comprising a monolayer of a sulfobetaine or a carboxybetaine material. In an embodiment, the sulfobetaine material is a well-defined diblock copolymer comprising a poly(sulfobetaine) and poly(propylene oxide). This co-polymer is formed in process comprising the steps of: synthesizing a poly(propylene oxide) macro-initiator (PPO-Br) by reacting monohydroxy-based poly(propylene glycol) with 2-bromoisobutyrylbromide in tetrahydrofuran; purification of that macro-initiator; sequential linking of sulfobetaine methacrylate monomer to the macro-initiator by atom transfer radical polymerization (ATRP); passing the resulting reaction solution through an aluminum oxide column; precipitating the product into ethanol and repeatedly re-dissolving said product into water to remove residual transition metal halide catalysts.
US Patent Application Publication No. 2011/0218290 A1 (Webster et al.) discloses the preparation of a zwitterionic/amphiphilic pentablock copolymer using atom transfer radical polymerization (ATRP). After removal of the copper bead or copper halide catalysts, the resultant pentablock copolymer is incorporated into a polyurethane coating composition which may find utility in antifouling and/or fouling release applications.
McCormick, et al., Polymer, 1992, 33, 4617 discloses a copolymer of acrylamide and the zwitterionic monomer 3-(2-acrylamido-2-methylpropanedimethylamino)-1-propanesulfonate. In this teaching the copolymers were prepared as homogeneous solutions in water but the polymerization was stopped at low conversion of monomer on account of the unmanageable viscosities attained.
There exists a need in the art to develop co-polymerization techniques that do not require the use of transition metal catalysts which have to be removed from the synthesized zwiterrionic copolymer prior to their use. Moreover, there is a need to develop a simple method—which should be capable to being scaled up such that it is performable at an industrial scale—for generating non-block zwitterionic copolymers in amounts appropriate for the large area surfaces with which fouling resistant coatings are concerned.